1. Field of the Invention
The present invention relates to a differential amplifier and a bias voltage generating circuit generating a bias voltage in the differential amplifier.
2. Description of the Background Art
For example, in a Japanese Patent Application Laid-Open No. 2002-124835 (2002), a composition of a general differential amplifier (in other words, an operational amplifier) including a differential amplifier circuit and a bias voltage generating circuit is described.
The differential amplifier circuit in the differential amplifier is generally composed of a current mirror circuit, two transistors connected respectively with two connect terminals of the current mirror circuit letting a reference current and a mirror current of a similar value to that of the reference current flow respectively and a constant current circuit connected with those two transistors in common (composed of one transistor generally). Moreover, the bias voltage generating circuit in the differential amplifier generates a bias voltage supplied to the constant current circuit in the differential amplifier circuit.
An input voltage signal and a reference voltage signal are inputted to the two transistors in the differential amplifier circuit, respectively. The differential amplifier detects a differential voltage between these signals, and amplifies and outputs this.
Besides, as for the other information disclosure statement corresponding to the invention of this application, a Japanese Patent Application Laid-Open No. 7-7340 (1995) is mentioned.
With regard to the conventional differential amplifier circuit and the bias voltage generating circuit as described above, there is a case that inconvenience occurs when a common mode voltage between the input voltage signal and the reference voltage signal decreases.
In other words, there is a case that a potential of common connect terminals of the two transistors decreases when absolute values of the respective voltages of the input voltage signal and the reference voltage signal decrease even if there is no change in a potential difference (the differential voltage) between the input voltage signal and the reference voltage signal and thus a value of a constant current flowing through the constant current circuit in the differential amplifier circuit decreases. When the constant current does not flow through the constant current circuit, it is impossible to detect the differential voltage correctly.